Attempt to call a nil

Not sure what I am doing wrong but the program wont even start this code is causing the error below

function updateSize()
  width, height = term.getSize()
  rows = math.floor((width - 1) / 25)
  offset = math.floor((width - rows * 25 + 1) / 2)
  surf = surface.create(width, height)
end

Can you please post your complete code and the full error you are getting?
Without seeing all the code I'd guess that surface.create is not defined.